Logic-based image processing method

ABSTRACT

A method for processing digital images to be displayed, stored, or printed, to eliminate blooming and other artifacts. The system utilizes morphological processes to isolate and modify image structures susceptible to marking process artifacts and then combines the modified image structures with the input image to produce a printable image that may be rendered on a given printer.

CROSS REFERENCE/INCORPORATION BY REFERENCE

The following related applications are hereby cross referenced andincorporated by reference for their teachings:

“USING MULTIPLE DIGITALLY-PRODUCED EXPOSURE LEVELS TO COMPENSATE FORLASER ABSORPTION IN READ IMAGE-ON-IMAGE XEROGRAPHY,” Henderson et al.,U.S. Pat. No. 6,111,593.

“METHOD AND APPARATUS FOR COMPENSATION OF BLOOMING ARTIFACTS,” Lin etal., application Ser. No. 09/219,276, filed Dec. 22, 1998.

“AUTOMATIC ENHANCEMENT OF PRINT QUALITY BASED ON FEATURE SIZE,” Eschbachet al., application Ser. No. 09/219,734, filed Dec. 22, 1998.

This invention relates generally to a logic-based image processingmethod for size dependent filtering and more particularly to logic-basedimage processing to compensate for marking process characteristics suchas blooming, and size and orientation dependent artifacts in axerographic engine.

BACKGROUND OF THE INVENTION

The present invention contemplates the use of logic-based, morphologicaloperations to isolate image structures requiring size dependentmodification, such as features that are susceptible to blooming whenreproduced by an output device such as a color xerographicimage-on-image, or any other type, of marking engine. The bloomingcondition, resulting from the need to overexpose the photoreceptor forlatter-developed colors that are imaged through an existing colorant,does not lend itself to correction by simple adjustment of xerographicparameters or simple color correction.

Heretofore, a number of patents and publications have disclosedlogic-based image processing, the relevant portions of which may bebriefly summarized as follows:

Crawford, J. L., and C. D. Elzinga, “Improved Output Quality byModulating Recording Power,” SPSE 41st Annual Conference, May 22-26,1988, Arlington, Va. Discusses utilizing thickened strokes whileperforming smoothing, and the use of logical mask processing.

Loce, R. and E. Dougherty, Enhancement and Restoration of DigitalDocuments, SPIE Press, Bellingham Wash., 1997. Provides much tutorialinformation oil logic-based image filtering and relevant morphologicaloperations—Section 1.5 teaches the basic relevant operations.

“Method and Apparatus for Digital Image Darkness Control Using QuantizedFractional Pixels,” Inventors: R. Bracco, et al., Ser. No. 09/072,122(May 5, 1997 provisional application, February 1998 actual filing),D/97210P,

Barski, L., and R. Gaborski, “Image Character Enhancement using a StrokeStrengthening Kernel,” U.S. Pat. No. 4,791,679, Dec. 13, 1988. Teacheshow a character stroke is strengthened by processing video image datawith a 16×16 kernel, and moving the kernel one pixel at a time throughthe image. For each pixel position, sections of the kernel, areselectively filled with black pixels in proportion to the number ofblack pixels in each section, in accordance with a set of predeterminedrules.

Crawford, J., and J. Cunningham, “Boldness Control in anElectrophotographic Machine,” U.S. Pat. No. 5,128,698, Jul. 7, 1992.Control over the placement of an image edge location on thephotoconductor of an electrophotographic machine as providing for arange of discharge levels for edge picture elements (PELS) which varyfrom greater than, to less than, that level used for fully dischargedPELS. Such control is achieved independently of machine parametercontrol by altering edge PEL illumination intensity in accordance withdata representing desired edge PEL intensity as the photoconductorsensitivity changes. A system for measuring and controlling the fullydischarged PEL level establishes a measure of photoconductor sensitivityand is used for enabling the selection of current edge PEL intensity.Used control of marking process parameters, as opposed to modifying thedigital image.

Mailloux, L., and T. Robson, “Dilation of Images without ResolutionConversion for Printer Characteristics,” U.S. Pat. No. 5,483,351, Jan.9, 1996. An image compensation system which provides dilation or erosionof image features using halfbitting or fullbitting in the rendition ofbitmap images, especially on a write-white printer. A region of pixelsof an image is isolated which includes two or more correctable pixellocations. A set of state determination rules, based on the formation ofpixels in the isolated region, is used to determine a corrected binarypixel state for each of the correctable pixels. Corrections for onecorrectable pixel may be considered in the state determination rules foradjacent correctable pixels. A single enhanced output pixel is providedfor each image input pixel, thereby preserving the original imageresolution. Performing enhancements on multiple input pixels. Teachesemployment of “halfbits” to thicken strokes by a factional amount whilemaintaining printer resolution.

Murata, K., “Image Processing Method and Apparatus,” U.S. Pat. No.5,450,208, Sep. 12, 1995. The image processing apparatus for smoothingedges in a reproduced image includes an image data generating circuitfor generating image data including a specified pixel and a plurality ofpixels surrounding the specified pixel; a sub-pixel data generatingcircuit for dividing the specified pixel included in the image data intoN sub-pixels, for detecting the condition of the specified pixel and thecondition of the plurality of pixels surrounding the specified pixelincluded in the image data by matching the image data with a pluralityof predetermined patterns, and for generating sub-pixel data fordetermining the number and position of sub-pixels to be exposed of the Nsub-pixels, based on the condition of the specified pixel and thecondition of the plurality of pixels surrounding the specified pixel;and supplying circuit for supplying the sub-pixel data to exposurecircuit which makes exposure. The sub-pixel data generating circuitgenerates sub-pixel data for exposing M sub-pixels of the N subpixels,when the specified pixel is detected to be an exposed pixel whichrequires no exposure correction, where M is smaller than N. Theinvention here relates to an image processing method and an imageprocessing apparatus for smoothing jagged edges of characters, etc., andfor stably reproducing thin lines and isolated dots, so as to achieve animage reproduction of high quality and to achieve an ideal tonecharacteristic by correcting the tone characteristic of digital halftoneimages.

SUMMARY OF THE INVENTION

Image-on-image (IOI) marking engines, where images are sequentiallyexposed and developed, typically produce a “blooming” artifact in thelater-imaged colors (e.g., magenta and cyan). Unfortunately, theblooming artifact does not easily lend itself to correction merely byadjusting the controls or setpoints of the xerographic engine. On theother hand, it has been discovered that it is possible to employ animage processing solution that will reduce the blooming artifact to anacceptable level. A morphological, or logic-based, image processingmethod may be employed to compensate for the loss of shadow detailassociated with an observed blooming artifact.

In an image-on-image xerographic marking engine, the magenta and cyanseparations are typically developed over the yellow separation. In IOI,the exposure level for magenta and cyan is therefore increased, comparedto the yellow separation exposure level, to compensate for thetransmission loss when the latter separation exposure occurs through thedeveloped yellow image (yellow toner). However, in regions of thephotoreceptor surface where there is no yellow toner, the exposure levelof the latter separations will likely be too high—resulting in excessiveline growth and loss of shadow detail called blooming. Blooming causes“holes” within a latent image to fill in with toner, which in turnresults in the loss of shadow detail in halftoned images.

Although it may be possible to correct the exposure intensity oil apixel-bypixel basis, the level of registration accuracy and hardwarecomplexity necessary to enable such a correction is not readilyachievable in commercial equipment. Similarly, other methods may beutilized to minimize the blooming artifacts. One such method, an objectof the present invention, is the use of logic-based non-linear or(morphological) methods to adjust the image bitmap in a manner thatcorrects for the loss of shadow detail. More specifically, the presentinvention is directed to the enlargement of certain features (e.g.,holes) within an image. Enlargement allows tile area about the “holes”to be exposed and developed, or printed, without filling in the holes.Generally, the invention first isolates the regions of the image mostlikely to be affected by blooming using a logic-based “sifting”operation, then processes the regions to compensate for blooming, andmerges or links the processed image with the original image to produce adigital image that will be blooming artifact reduced when printed withan image-on-image marking engine.

While blooming contributes to a particular size dependant artifact,other marking process attributes contribute to other size dependentartifacts. For instance, a marking process may print with narrow darklines that are too thin, or wide dark lines that are too thin, or linesof a particular width in a given orientation are too thin or too thick.Some combination of size and orientation dependent artifacts may occur.A key aspect of the present invention is the generalized process of sizeand orientation dependent filtering for compensation of such artifacts,or for generation of a preferred rendition. This generalized operationmay be thought of as a sieving operation, where image features of aparticular size and orientation are sieved into classes, each class ismodified, and the modified feature is input to the final processedimage.

Another aspect of the invention is based on the recognition of theproblem of blooming artifacts that appear in a display or other markingengine like an image-on image xerographic printing system. Moreparticularly, the invention utilizes image processing techniques tocompensate for the blooming artifacts. The techniques employlogic-based, morphological filters to identify structures susceptible toblooming and other printing artifacts and then modify the structures tocompensate for those artifacts before the digital image is rendered.

In yet another aspect of the invention, in addition to compensation formarking process characteristics, the present invention performs size andorientation filtering to produce an image that may produce a preferredrendition, possibly for some psychophysical reason or preference.

It will be well understood by those skilled in the art that thecompensated image may be written not only to a printing device ormarking engine, but may also be written to a display device or to astorage device for subsequent retrieval and use.

In accordance with yet another aspect of the present invention, there isprovided a method for processing a digital image prior to printing theimage, including the steps of isolating regions of the digital imagemost likely to be affected by blooming or other size-dependent markingartifacts, modifying the isolated regions to compensate for thoseartifacts; and merging the modified image features with unmodified imagefeatures in the digital image to produce an output digital image that isartifact free when printed using an marking engine, such as animage-on-image device.

In accordance to the present invention, there is provided a method forprocessing a digital image prior to printing the image on a givenmarking engine, including: isolating regions of the digital image mostlikely to be affected by an undesirable printing characteristic;producing a modified image by modifying the isolated regions tocompensate for the undesirable printing characteristic; and merging themodified image with the digital image to produce an output digital imagethat is artifact free when printed using the given marking engine.

In the alternative, there is disclosed a method for processing a digitalinput image prior to printing the image on a given marking engine,including the steps of: isolating features that are not to be subject toa particular modification, modifying the remainder of the image with amorphological filter, and merging together the modified and unmodifiedfeatures. For example, suppose it is necessary to thin most features ofan image to compensate for blooming. Fine foreground feature regions ofthe digital input image would be adversely affected by such acompensation operation. In this case, a morphological filter is used toisolated the fine foreground filters, and a second morphological filter,such as an erosion is used to perform the compensation on the remainingimage features. Then a subsequent, merging operation is performed tocombine the unmodified fine foreground features with the compensatedimage features, thereby producing an output digital image that isartifact free when printed using the given marking engine.

These techniques can be implemented with a machine that prints colorimages from digital image data, including digital printers such as laserprinters and facsimile machines. These techniques may also be utilizedfor displays or to images which are subsequently stored for later use.The technique described above is advantageous because it is efficientand can be accomplished using programmable image processing components.It is also flexible and can be adapted to compensate for any of a numberof image structures that may result in visually perceptible printingartifacts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a generalized, schematic illustration of a printing systemembodying the present invention;

FIG. 2 is a block diagram showing particular detail of thefilter/processing components within the printing system of FIG. 1;

FIGS. 3 and 4, respectively, illustrate serial and parallel embodimentsof the present invention;

FIG. 5 is a generalized block diagram representing a processing systemsuitable for processing both positive and negative image structures;

FIG. 6 illustrates an exemplary embodiment of the present inventiondesigned to isolate fine foreground image features and hole structurethereby eliminating undesirable marking characteristics and bloomingartifacts found in marking engines;

FIG. 7 illustrates the structuring elements required for the operationof the invention as embodied in FIG. 6;

FIG. 8 illustrates an exemplary embodiment of the present invention asdesigned to isolate only fine foreground image features, therebyeliminating undesirable marking characteristics and blooming artifactsfound in marking engines; and,

FIG. 9 illustrates the structuring elements required for the operationof the invention as embodied in FIG. 8.

The present invention will be described in connection with a preferredembodiment, however, it will be understood that there is no intent tolimit the invention to the embodiment described. On the contrary, theintent is to cover all alternatives, modifications, and equivalents asmay be included within the spirit and scope of the invention as definedby the appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT

For a general understanding of the present invention, reference is madeto the drawings. In the drawings, like reference numerals have been usedthroughout to designate identical elements. In describing the presentinvention, the following term(s) have been used in the description.

An “image” is a pattern of physical light. An image may includecharacters, words, and text as well as other features such as graphics.An image may be divided into “segments” or “regions”, each of which isitself an image. A region of an image may be of any size up to andincluding the whole image.

An item of data “defines” an image when the item of data includessufficient information to produce the image. For example, atwo-dimensional array can define all or any part of an image, with eachitem of data in the array providing a value indicating the color of arespective location of the image.

Each location in an image may be called a “pixel. A “pixel” is thesmallest segment of an image whose value is indicated in an item of datadefining the image. In an array defining an image in which each item ofdata provides a value, each value indicating the color of a location maybe called a “pixel value”. Each pixel value is a bit in a “binary form”of an image, a gray scale value in a “gray scale form” of an image, or aset of color space coordinates in a “color form” of an image, the binaryform, gray scale form, and color form each being a two-dimensional arraydefining an image.

An operation performs “image processing” when it operates on an item ofdata that relates to part of an image. A “morphological” or“logic-based” operation operates using logical operators (e.g., AND, OR,INV, NOT) applied to a digital image. In particular, the logicoperations are typically applied in association with a “structuringelement” such as an aperture having a predefined shape or other set ofcharacteristics.

An “edge” occurs in an image when two neighboring pixels havesufficiently different pixel values according to an appropriatecriterion for the occurrence of an edge between them. The term “edgepixel” may be applied to one or both of two neighboring pixels betweenwhich an edge occurs.

An “image characteristic” or “characteristic” is a measurable attributeof an image. An operation can “measure” a characteristic by producingdata indicating the characteristic using data defining an image. Acharacteristic is measured “for an image” if the characteristic ismeasured in a manner that is likely to produce approximately the sameresult each time it occurs.

A “version” of a first image is a second image produced using an item ofdata defining the first image. The second image may be identical to thefirst image, or it may be modified, such as by image processingoperations.

An “image input device” (IIT) is a device that can receive an image andprovide an item of data defining a version of the image. A “scanner” isan image input device that receives an image by a scanning operation,such as by scanning a hardcopy document. An “image output device” (IOT)is a device that can receive an item of data defining an image andprovide the image as a visual output. A “xerographic marking engine” isan image output device that provides the output image in hardcopydocument form.

A number of morphological operations map a source image onto an equallysized destination image according to a rule defined by a pixel patterncalled a structuring element (SE). The SE is defined by a centerlocation and a number of pixel locations, each having a defined value(ON or OFF for the binary case, with Grey-scale morphology allinter-mediate levels are allowed). The pixels defining the SE do nothave to be adjacent each other. The center location need not be at thegeometrical center of the pattern; indeed it need not even be inside thepattern.

“Erosion” is a morphological operation wherein a given pixel in thedestination image is turned ON if and only if the result ofsuperimposing the SE center on the corresponding pixel location in thesource image results in a match between all ON pixels in the SE and Onpixels in the underlying pixels in the source image.

“Dilation” is a morphological operation wherein a given pixel in thesource image being ON causes the SE to be written into the destinationimage with the SE center at the corresponding location in thedestination image.

“Opening” is a morphological operation that can be represented as anerosion followed by a dilation. The result is to replicate the SE in thedestination image at each location in which it fits within the sourceimage.

“Closing” is a morphological operation that may be represented as adilation followed by an erosion. It may also be thought of as an openingperformed on the background of an image.

Turning to FIG. 1, depicted therein is a generalized, schematicillustration of a printing system embodying the present invention. Inprinting system 16 (which includes digital reprographic systems) adigital image is initially obtained from an image input terminal or IIT18. IIT 18 may be any device suitable for generating or storing adigital image for reuse, including for example, a storage device 20 suchas a magnetic disk, a scanner or similar digital imaging device 22(e.g., digital camera), or a computer or similar networked imagecomposition device 24.

Digital input image 30, having been obtained from IIT 18, is passed toall image processor 32 where it is processed in accordance withpre-programmed instructions. Image processor 32 may be any suitablehardware device, particularly including those devices designed fordigital signal processing. Preferably, however, image processor 32 is aprogrammable hardware device capable of operating on the input imagedata in a timely fashion to meet the throughput requirements of the IOTthat it supports. Image processor 32 also contains blooming controllogic 34 (hardware and/or software) that will be described in furtherdetail with respect to the following figures.

The output of image processor 32, in the form of a processed digitalimage, is preferably passed to a marking engine or IOT 36. In apreferred embodiment, IOT 36 is an image-on-image, xerographic engine ascharacterized above. Simply put, the marking engine exposes and developssequential color separations (for example: yellow, magenta and cyan) oneon top of the other. Such a system inherently requires that for anycolors that are formed as a combination of two primary colors, theremust be repeated exposures of certain regions of the photoreceptor inorder to develop the color image region. Once exposed and developed, theimage on a photoconductive member (not shown) is transferred andpermanently affixed to a substrate, represented as output print 38.

Referring to FIG. 2 there is shown particular detail of thefilter/processing components within the printing system of FIG. 1. Inparticular, blooming filter/processor 34 is indicated as including amorphological or logic-based isolation operation 40 and an imageprocessing operation 42. Furthermore, morphological isolation operation40 receives as input not only the input image 30, but at least onestructuring element 44. Exemplary examples of structuring elements willbe described below in the description of FIGS. 7 and 9. Those skilled inthe art of image processing will recognize the need for a structuringelement in the logic-based operations of a blooming filter/processor.Although it is possible to represent the blooming filter/processor asits individual component operational elements, the following descriptionwill simply characterize the operations as filter/process 34.

The morphological or logic-based operations of block 40 are intended toidentify or isolate particular structures within a digital image. Forexample, the structures could be lines or holes that need to be furtherprocessed to avoid the appearance of blooming artifacts and otherundesirable marking process characteristics. Other examples mightinclude, filiform, and filigree as found in the background or foregroundof an image; sharp features such as the serifs found in various fonts,or the need to adjust or enlarge ink traps of various fonts. Emphasismay be made of horizontal or vertical lines, or lines which lay in thedirection of the marking process direction, or as either parallel orperpendicular to the marking process direction. Determination of whichstructures are isolated is accomplished by the choice of one or morestructuring elements 44. Once isolated, the pixels representing thestructure and possibly its surrounding, are then processed in accordancewith an image processing operation specifically intended for thestructure. For example, a morphological operation may be employed toidentify single-pixel holes within an image. Because blooming causessuch holes to be filled by the IOI marking process, the image processingoperation to be performed would preferably be an erosion operation thatwould erode the area surrounding the hole, hence slightly expand thesize of a hole. Morphological operations such as erosion, dilation,opening, and closing, are well known and discussed, for example, in U.S.Pat. No. 5,048,109 to Bloomberg, incorporated herein by reference. Onceprocessed in accordance with the image processing of operation 42, aprocessed output image 46 is generated and provided for transmission toa print engine, for further processing.

Considering FIGS. 3 and 4, the figures respectively illustrate serialand parallel embodiments of the present invention. FIG. 3 illustrates aserialized flow of input image data through multiple filter/processoperations 34A, B, ultimately being 15 recombined by recombination logic48, which is typically Boolean logic, in order to generate a printableoutput image 50. FIG. 4 is a 3-channel representation of a parallelfilter/process embodiment, were the isolation and processing operationsoccur concurrently in filter/process operations 34A, B and C, before theoutputs thereof are recombined by recombination logic 48.

FIG. 5 is a generalized block diagram representing a processing systemsuitable for processing concurrently both positive and negative imagestructures. The need to process both positive and negative structuresarises as a result of the blooming effect. While a hole (a negativeimage structure where no image is exposed/developed) may be susceptibleto being filled in by blooming, it is also possible for a positiveforeground structure within the image (e.g., a 1-pixel wide line) to beundesirably expanded by blooming. As represented by FIG. 5, there may bea plurality of filter/process operations (34A-F) to identify and processparticular structures of interest. Some of the filter/process operationsmay be directed to positive image structures (34A-34C), whereas othersmay be directed to negative image structures (34D-34F). Again, theoutputs of each filter/process operation are recombined, in combinationwith the input image data in a recombination logic process 48, toproduce a final output image 50 that compensates for the bloomingeffects.

Referring now to combination FIGS. 6 & 7 and 8 & 9, there will beexplained two exemplary embodiments of the present invention, designedto eliminate particular characteristics of blooming found inimage-oil-image marking engines. Note that the open circle symboldenotes “open” the closed circle symbol denotes “close” and the trianglesymbol denotes “difference”.

FIG. 6 illustrates a method for the enlargement of holes to prevent theloss of shadow detail. Specifically, input image 30 is first processedin accordance with a logic-based or morphological operation 60A,B thatis designed to employ “opening” (60A) and “closing” (60B) so as toseparate both the holes and the fine foreground features from otherstructures within the image. As a result, the fine foreground structureimage 62 is left after an “open” and difference operation, whereas thehole image 64 remains after a “close” and difference operation. Theimage of holes 64 is subsequently processed by an erosion or dilationoperation 68 (dilation in the present example due to the givenembodiment representing a hole with 1 value at this stage) so as toproduce an enlarged hole image 70. The enlarged hole image 70 is thenlogically recombined, using recombination logic 48, with the fineforeground structure image 62 and original input image 30, so as toproduce all output image 50 with enlarged holes.

FIG. 7 illustrates the three logic-based structures employed to isolatethe various image structures. The FIG. 7 structuring elements K₁, K₂,and B, are depicted as pixel groupings. K₁ is the structure used withthe close operation to isolate holes, as represented logically by theequation (AK₁) Δ A. K₂ is the structure used with the open operation toseparate fine foreground structure as represented by the equation A Δ(A∘K₂). Similarly, B is the structure employed to dilate the holes asrepresented by the logic equation A₀⊕B. Thus, when used in combinationwith the process of FIG. 6, the FIG. 7 structuring elements enable theisolation of holes and fine foreground features prior to hole growth inthe digital image.

Turning next to FIGS. 8 and 9, there is shown a similar exemplaryprocess and associated structuring elements. In particular, FIG. 8 showsthe detail of an alternative method employing opening and erodingoperations to isolate fine foreground structures from holes prior tohole growth. Initially input image 30 is morphologically “opened” toremove holes at step 80A. Using the logic represented by the equationA₁=AΔ (A∘K), resultant image 82 is generated using the structuringelement K depicted in FIG. 9. In either a subsequent or concurrentfashion, the input image is also morphologically processed to erode theimage and thereby enlarge existing holes as represented by step 80B. Asindicated by the following logic equation, A_(OB)=A⊖B, the eroded image84 is created using the structuring element B as illustrated in FIG. 9.

Once both intermediate images 82 and 84 are generated, they arelogically OR'ed at step 48 to recombine the intermediate images and thusyield output image 50. Output image 50 will thereby have enlarged holesthat are less susceptible to being filled as a result of bloomingartifacts caused by an IOI marking engine used to render the image. Itshould also be noted that, as depicted by the structuring elements ofFIGS. 7 and 9, the previously-described processes have particularapplicability in a fast-scan, high-addressability (e.g., 4X) markingsystem, where each pixel consists of four sub-pixels that may beindividually controlled so as to produce a high-resolution output.

The hole-growth methods described herein are particularly applicable toa number of image processing display and printer architectures. Asmentioned, the hole-growth blooming compensation operations would beperformed on an image bitmap after it has been rendered or halftoned toa high-addressability form. The structuring elements shown in FIGS. 7and 9 may be suitably employed for such an architecture. However, in analternative architecture, the hole-growth compensation techniques mayalso be employed on lower resolution, thresholded images, and theaffected or modified regions could be merged into a high-addressabilityversion of the image generated by a parallel processing path. Forexample, a two-channel processing system could be employed, where onechannel performs resolution enhancement on high-contrast, saturatedstructures like line art and text. The holegrowth techniques describedherein could be incorporated within the resolution enhancement channel.Such an architecture would be beneficial because the holegrowth bloomingcompensation operations are particularly suited for improving theprinted rendition of high-contrast, saturated edges.

In yet another embodiment, it may be possible to apply analogousprocessing techniques directly to a multi-bit grayscale color separation(e.g., 8-bit multi-valued pixels). In such an embodiment, thelogic-based OR and AND operations would be replaced with MAX and MINoperations respectively. As is known in the art of morphological imageprocessing, there are gray-scale operations that are equivalent to thebinary operations. The ideas described above naturally extend toimplementations on gray-scale images via use of gray-scale morphologicaloperators. For instance, a hole in a binary image is defined as a smallregion of zero-valued pixels surrounded by one-values pixels. Astructuring element that finds such a hole is of comparable size and isvalued one or zero. For a gray-scale image, a hole may be a small regionof low pixel values in a field of pixels possessing higher values.Gray-scale structuring elements would possess a comparable gray-range tothe input data and be of comparable size. Upon sieving a gray-scaleimage for its features of interest, the modifications could be similarto those performed in the binary setting, that is, an increase ordecrease in size, and also the modifications may be a change in pixelintensity through a value mapping operations. A summary of such anoperation is as follows. Assume the use of gray-scale morphology toidentify gray holes. Once identified, the values within the localdepression could all be set to chosen value, such as zero. Resetting tosuch a value results in more contrast in the final image.

As is well known in the art, morphological operations performed alone orin combination can be implemented and represented in various forms, suchas Boolean logic, look-up tables, and nonlinear filters. For instance,the above operations may be implemented using sliding window operatorsand look-up tables. Where a window of pixels are observed about a targetand those pixels are used as an index to a look-up table that generatesthe output signal for that pixel in accordance with the abovemorphological operations. Further, combinations of the above-describedoperations may be implemented in various forms, just as multiple Booleanlogic equations may be combined and written in various equivalent forms,including truth tables.

In recapitulation, the present invention is a method and apparatus forlogic-based image processing of a digital image to compensate for orreduce process artifacts that occur when the image is displayed, orprinted by a marking engine such as an image-on-image xerographicengine. In particular, there is described a method for processing adigital image, including the steps of isolating regions of the digitalimage most likely to be affected by marking artifact, modifying theisolated regions to compensate for the artifact; and merging themodified image with the digital image to produce an output digital imagethat is artifact reduced when printed using an image on-image markingengine.

It is, therefore, apparent that there has been provided, in accordancewith the present invention, a method and apparatus for logic-based imageprocessing. While this invention has been described in conjunction withpreferred embodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart. Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

We claim:
 1. A method for processing a digital image, including:isolating regions of the digital image including those regions mostlikely to be affected by an undesirable display or printing artifact;producing a modified image by modifying at least one of the isolatedregions to compensate for the undesirable display or printing artifact;and merging the modified image with the digital image to produce anoutput digital image that is artifact reduced.
 2. The method of claim 1,wherein isolating regions of the digital image most likely to beaffected by undesirable printing artifact includes applying amorphological filter to the digital image.
 3. The method of claim 1,wherein isolating regions of the digital image most likely to beaffected by undesirable printing artifact includes applying a nonlinearfilter to the digital image.
 4. The method of claim 2, wherein themorphological filter identifies a positive image structure.
 5. Themethod of claim 4, wherein the image structure are fine image features.6. The method of claim 4, wherein the positive image structure is one ofa group of image structures, the group consisting essentially of: ahorizontal line; a vertical line; a line at a particular angle; a lineparallel to the marking process direction; a line perpendicular to themarking process direction; a line of a particular width; a dot; and aserif.
 7. The method of claim 2, wherein the morphological filteridentifies a negative image structure.
 8. The method of claim 7, whereinthe image structure are fine image features.
 9. The method of claim 7,wherein the negative image structure is one of a group of imagestructures, the group consisting essentially of: a horizontal line; avertical line; a line at a particular angle; a line parallel to themarking process direction; a line perpendicular to the marking processdirection; a line of a particular width; a hole; and a serif.
 10. Themethod of claim 1, wherein there are a plurality of isolating steps,each isolating step isolating a region within the image containing aparticular structure; and where there is a particular modifying step foreach of said particular structures.
 11. The method of claim 10, whereinthe isolating steps are able to identify positive and negative imagestructures in parallel.
 12. The method of claim 2, wherein applying amorphological filter is as a closing operation.
 13. The method of claim10, wherein the step of modifying the isolated regions includes adilation operation.
 14. The method of claim 2, wherein applying amorphological filter is as an opening operation.
 15. The method of claim2, wherein the step of modifying the isolated region includes an erosionoperation.
 16. A method for processing a digital image prior toprinting, displaying, or storing the image including: isolating sizedependent regions of the digital image most likely to be affected by anundesirable printing artifact with a morphological filter utilizing astructuring element; producing a modified image by modifying at leastone of the isolated size dependent regions to compensate for theundesirable printing artifact; and, merging the modified image with thedigital image to produce an output digital image that is artifactreduced when printed or displayed.
 17. The method of claim 16, whereinthe size dependent regions are very small fine features.
 18. The methodof claim 16, wherein the size dependent regions are holes which arefilled in by blooming.
 19. The method of claim 16, wherein thestructuring element is so chosen as to filter for fine image features.20. The method of claim 16, wherein the structuring element is so chosenas to filter for one of a group of image structures, the groupconsisting essentially of: a horizontal line; a vertical line; a line ata particular angle; a line parallel to the marking process direction; aline perpendicular to the marking process direction; a line of aparticular width; a dot; a hole; and a serif.
 21. A method forprocessing a digital input image prior to printing, displaying, orstoring the image, including: isolating feature regions of the digitalinput image most likely to be adversely affected by a modificationoperation of an undesirable printing or display artifact; modifying thedigital input image to produce an modified image; and, merging theisolated feature regions of the digital input image with the modifiedimage to produce an output digital image that is artifact reduced whenprinted or displayed.
 22. The method of claim 21, wherein the digitalinput image is built of single-bit, zero or one value pixels.
 23. Themethod of claim 21, wherein the digital input image is built ofmulti-bit, multi-value pixels.